Fractured bones and corrective osteotomies are surgically treated by means of one of the three generic medical device families: (1) plates and screws; (2) intramedullary nails, and (3) external fixators. Selection of external fixation by trauma and orthopedic surgeons is greatly influenced by their formation, attitude, place of practice and general economic conditions in addition to the nature of the medical problem to be treated. This greatly complicates any estimates of what the number of cases being treated by any of the methods might be worldwide, but all three are universally considered as fundamentally important surgical aids. Internal fixation by plates or nails is simply not possible in most of the undeveloped world, thus making external fixation the only potentially viable, modern alternative to complement conservative treatments by fracture splinting or casting. Unfortunately, the cost of external fixators produced in the developed world is also prohibitive for most of the undeveloped countries.
The main drawback of external fixation is also rather uncertain and uneven progress of bone healing when compared to internal fixation. Passage of bone pins or wires through soft tissue surrounding the bone and the skin increases the risk of infection when the fixator is kept on the patient for months. Much of the uncertainty can be explained in view of rather recent research findings, which have detailed different biological phases of fracture healing that call for different mechanical conditions at the fracture site. In the very early stages, in the first week or two, as the repair is initiated, differentiating factors emanating from the surrounding bone need to provide signals to the proliferating cells in the fracture zone to actually turn themselves into bone forming cells. In absence of any movement across the fracture, or the osteotomy, mass transport of the signals out of the bone and throughout the zone of repair is limited to diffusion, which may not suffice when the gaps are in excess of some hundreds of micrometers. Formation of fibrous tissue in the gap, which is what tends to quickly fill any tissue defects, may slow down, if not totally frustrate formation of bone proper, leading to delayed unions or non unions. Movement and particularly compression of the gap promotes convective mass transport of the biological factors out of the bone and within the gap, driving the differentiation process towards bone formation.
After this early period, however, once the early organic matrix for bone has been produced and the mineralization starts to set in, excessive movement will prevent bridging of the gaps between the nuclei of mineralization and disrupt the healing process. So now, in order to facilitate a safe and thorough process of mineralization, the movement across the gap should be reduced as much as possible calling for as stiff a construct of the external fixator as possible.
In the final stages of fracture union, presence of the stiff external fixator may hinder remodeling of healed bone by reducing the full physiological loading it will need to support once the fixator is removed, so again, a change in fixator stiffness, back to low, is called for.
There are innumerous ways in which this modulation can be carried out with conventional external fixators, but they all suffer from deficiencies. The fixator of this invention is particularly suited to overcome these deficiencies.